Sometimes a dispatcher or an admin may need to complete a tour on behalf of a driver. It may happen when a driver hasn't completed his tour (completed all tasks but failed to tap the 'End Tour' button in his driver app) and is not available the next day or several days after the tour was executed.
In this situation, you can select this tour and click on three dots in the upper right of the Assignments sections, and select Complete tour from the dropdown. In the pop up that will show up, you may additionally select the exact time when you would like the tour should be ended. When done click Confirm - the tour will be moved to Done and will be considered as completed.